1. Field of Invention
This invention relates to a metallic pattern forming method, specifically relates to a metallic pattern forming method that provides metallic patterns which can be used as metallic wiring pattern substrate or print wiring pattern substrate having dimensional stability, and further relates to a conductive pattern material that is obtainable by the method.
2. Description of the Related Art
As conventional methods of forming metallic patterns, a “subtractive method”, a “semi-additive method” and a “full-additive method” are mainly known.
The subtractive method is a method in which a photosensitive layer reactive to irradiation of active light is provided on a metallic layer formed on a substrate, the photosensitive layer is subjected to image-wise exposure, the image is developed so as to form a resist image, the metal is etched so as to form a metallic pattern, and, as a final step, the resist is stripped. In the metallic substrate used in the above-mentioned method, an adhesion property is generated by means of an anchor effect resulting from applying an unevenness process to an interface between the substrate and the metallic layer in order to adhere the substrate and the metallic layer to each other. As a result, the interface part of the finished metallic pattern relative to the substrate is rendered uneven, which unfavorably deteriorates a high-frequency characteristic when the metallic pattern is used as an electric wiring. As another problem, when the metallic substrate is formed, the substrate is subjected to the unevenness process, which requires a complicated step of treating the substrate with strong acid such as chromic acid.
In order to eliminate the above-mentioned problems, a method of simplifying the processing steps for the substrate by grafting a radically-polymerizable compound on a surface of the substrate so as to modify a property of the substrate surface (as examples of which, see Japanese Patent Application Laid-Open (JP-A) No. 58-196238, and pp 1481-1494 of “Advanced Materials”, 20th edition, published in 2000) has been proposed. The metallic substrate formed in the above-mentioned method can be patterned by means of the subtractive method, but, the subtractive method has its own unique problem, that is, a so-called over-etching process in which a post-etching line width becomes thinner than a line width of a resist pattern is advantageous in order to form the metallic pattern having a fine line width by means of the subtractive method (for example, see JP-A No. 2004-31588). The reason why the over-etching process is problematic is that the formation of the fine metallic pattern directly by the over-etching process easily leads to the generation of a blurred line, a faint line, a broken line and the like, making it difficult to form a metallic pattern of 30 μm or less, which is disadvantageous for the formation of a favorable fine metallic pattern. The above-mentioned conventional method further creates a problem, from the standpoint of the environmental and pricing, because the metallic film present in any area other than the pattern portion is removed and therefore wasted in large amounts by the etching process and because the disposal of waste fluid generated by the etching process is costly.
In order to eliminate the above-mentioned problems, the metallic pattern forming method called the semi-additive method has been proposed. The semi-additive method is a method in which a thin ground substrate layer made of Cr or the like is formed on the substrate by means of plating or the like, a resist pattern is formed on the ground metallic layer, a metallic layer made of Cu or the like is formed on the ground metallic layer other than the region where the resist pattern is formed by means of the plating, the resist pattern is removed so as to form a wiring pattern, the wiring pattern is used as a mask to etch the ground metallic layer, and a metallic pattern is formed in the region other than where the resist pattern is formed. The method does not require the etching process, and therefore, is capable of easily forming a fine line pattern of 30 μm or less. The method is also effective from the standpoints of the environment and pricing because the metal is deposited only in a desired part by means of the plating. However, there is also a problem included in this method in that it is necessary to apply the unevenness process to the substrate surface in order to generate the adhesion property between the substrate and the metallic pattern, as a result of which the interface of the finished metallic pattern becomes uneven relative to the substrate and the high-frequency characteristic thereby deteriorates when the metallic pattern is used as an electric wiring.
The metallic pattern forming method called the full-additive method has also been proposed. The full-additive method is a method in which a resist pattern is formed on the substrate, the metal is deposited other than the region where the resist pattern is formed, and the resist pattern is thereafter removed. This method, which is also an etchingless method, enables the easy formation of a fine line pattern of 30 μm or less, but shares the same problem as in the case of the semi-additive method. Therefore, a novel metallic pattern forming method capable of forming a thin line pattern, reducing unevenness in the substrate interface, and reducing the waste fluid generated by the etching process is desired.
The substrate used for the formation of the wiring substrate is necessarily solder-resistant and is usually required to have heat resistance against approximately 250° C. Therefore, a polyimide substrate having a superior heat resistance is generally used. However, the problems associated with the metallic pattern forming mentioned earlier remain unsolved even in using the polyimide substrate. Thus, a novel metallic pattern forming method is also desired for the formation of a metallic pattern on a polyimide substrate, which is superior in heat resistance. However, the current situation is that such a method has not yet been provided.
Also, when a polyimide substrate is used in conductive material applications such as flexible wiring, TAB (Tape Automated Bonding) tapes, laminated wiring substrates or the like, it is required that the moisture absorption ratio of the polyimide is as small as possible from the perspectives of reliability and dimensional stability. Again from the perspective of dimensional stability it is also required that the hygroscopic expansion coefficient is also low. This is because during electrical wiring type manufacturing processes there are repeated cycles of processes, such as washing/drying, where the polyimide substrate absorbs water and then is dried out. Consequently, as a result of moisture absorption/moisture loss in the base film, there are large changes in the dimensions of the metallic wiring parts, and this can lead to errors when mounting an IC chip or the like. It is also because when heat is added during mounting, the moisture content in the polyimide can evaporate, leading to large dimensional changes, which again can lead to the occurrence of errors. Also, since polyimide substrates are also used in application where they are bent or folded, a high degree of elasticity is required from the perspective of flexibility.
Regarding polyimides exhibiting high elasticity, such as those described in Japanese Patent Application Laid-Open (JP-A) No. 200346223 and in “Macromolecules” No. 29, p 1642 to p 1648 by N Inagaki, S Tasaka, and M Masumoto, if, for making the main chain of the polyimide, a general acid of pyromellitic dianhydride is used as the material to synthesize the polyimide, then the exhibiting of high elasticity can easily be achieved. However, polyimides which are obtained like this cannot exhibit low hygroscopicity because the polarization of the imide groups is high.
In order to overcome this problem, it is generally effective to reduce the quantity of imide groups in the molecular structure, and the use of flexible groups of long monomer chains within the main chain is common. However, if simply the number of imide groups in the molecular structure is reduced, then this can lead to a reduction in the elasticity and an excessive increase in the linear expansion coefficient, sacrificing dimensional stability. Also, another problematic point about the characteristics of conventional polyimides is that, if long linear monomers are used, then the molecular chain packing becomes difficult, and it is difficult to achieve sufficient toughness, and in some case it is difficult to form films. Like the above, the required characteristics of a polyimide need to consider lots of other perspectives as well as low linear expansion coefficient, reducing hygroscopicity, and increasing elasticity. However, the current situation is that if one characteristic is satisfied another is sacrificed, and obtaining a polyamide film possessing all of multiple good characteristics posses significant problems, with an influence on multiple-functions of conductive patterning materials to which polyimide films are applied.